Over the last few decades, the electronics industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly integrated electronic devices. The most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applicability and numerous disciplines. One such silicon-based semiconductor device is a metal-oxide-semiconductor (MOS) transistor.
A typical MOS semiconductor device generally includes a gate electrode, which acts as a conductor, to which an input signal is typically applied via a gate terminal. Heavily doped source/drain regions are formed in a semiconductor substrate and are respectively connected to source and drain terminals. A channel region is formed in the semiconductor substrate beneath the gate electrode and separates the source/drain regions. The channel is typically lightly doped with a dopant type opposite that of the source/drain regions. The gate electrode is physically separated from the semiconductor substrate by a gate insulating layer, typically an oxide layer such as SiO.sub.2. The insulating layer is provided to prevent current from flowing between the gate electrode and the source/drain regions or channel region.
In operation, an output voltage is typically developed between the source and drain terminals. When an input voltage is applied to the gate electrode, a transverse electric field is set up in the channel region. By varying the transverse electric field, it is possible to modulate the conductance of the channel region between the source region/drain regions. In this manner an electric field controls the current flow through the channel region. This type of device is commonly referred to as a MOS field-effect-transistors (MOSFET).
Semiconductor devices, like the one described above, are used in large numbers to construct most modern electronic devices. In order to increase the capability of such electronic devices, it is necessary to integrate ever increasing numbers of such devices into a single silicon wafer. As the semiconductor devices are scaled down (i.e., made smaller) and in order to form a larger number of devices on a given surface area, the structure of the devices and fabrication techniques used to make such devices must be altered.
One important step in the manufacture of MOS devices is the formation of the gate insulating layer. The gate insulating layer is typically formed by growing an oxide, typically SiO.sub.2, over the surface of the substrate. It is important to carefully control the growth of the gate oxide layer because the thickness and uniformity of the gate oxide layer can significantly impact the overall operation of the device being formed. For example, the drive current in a MOS transistor is inversely proportional to the gate oxide thickness at a given set of terminal voltages. Accordingly, it is normally desired to increase the drive current of the transistor by making the gate oxide as thin as possible, taking into consideration the oxide breakdown and reliability considerations of the process and technology being used.
The above described conventional techniques for forming gate oxide layers impose limitations on the minimum thickness of the gate oxide layer and on the ability to control the uniformity of the gate oxide layer. As the thresholds for minimum thickness and uniformity control are reached, the ability to further scale down the semiconductor devices is hindered.